This invention relates to security systems and in particular, but not exclusively, to a security system suitable for use in a vehicle.
It is known to provide a protected area such as a building or a vehicle with a security system which includes a passive entry feature and one system of this type is disclosed in EP 0 218 251. Such passive security, systems may, in addition or in the alternative, be applied to the starting system off a vehicle to provide passive re-mobilisation of an engine. Interference from unrelated radio frequency sources can cause problems for systems of this type.
It is also known to transmit signals from a portable transponder in more than one frequency range. One arrangement of this type is shown in EP 0440974, in which the transponder is used to perform different functions in each range. In one range it sends security signals to the vehicle and in a second range it is operative to open remotely a garage door.
In circumstances where there is a disruption in the normal operating conditions of some types of security system, it is possible to vary parameters of a signal from a remote transponder between successive transmissions thereof. One system in which this occurs is disclosed in GB 2282252, and in this system parameters of the response signal are altered on the basis of measured values, i.e. received signal strength and/or measured battery power. It is a possible disadvantage of this arrangement that it is necessary to provide means in the remote transponder to determine the level of these values and this might add complexity and expense to the system.
It is an object 6f this invention to provide an improved security system.
According to the invention there is provided a security system comprising a security control means and a remote transponder which are arranged in use to communicate in a range of channels using command and response signals transmitted in one or more of said channels, said security control means being arranged in use to transmit a said command signal in a said channel, said remote transponder being arranged in use, in response to a receipt of said command signal, to transmit a said response signal initially in an initial said channel, said security control means being arranged, if it does not successfully receive a said response signal, to re-transmit said command signal, wherein said remote transponder is arranged to determine if a command signal is a re-transmitted command signal and, if it determines that a said command signal is a re-transmitted said command signal, to re-transmit said response signal in one or more of said channels which is/are different to said initial channel.
Said remote transponder may be arranged to determine that a particular said command signal is a re-transmitted said command signal if it receives, that particular said command signal within a time slot of a predetermined duration which commences on or after it has received an earlier said command signal.
Said security control means may be arranged, to modify said command signal before re-transmitting it, such that said modified re-transmitted command signal includes: information identifying it to said remote transponder as a re-transniitted said command signal.
Said information may be indicative of which different said channel or channels is or are to be used to re-transmit said response signal in reply to said re-transmitted command signal.
Saidxe2x80x9cchannel of transmission of said command signal may operate in the Low Frequency band (LF).
Said initial channel of transmission of said response signal may operate in the Low Frequency band (LF).
In response to the detection of a re-transmitted said command signal, said response signal may be re-transmitted in a said channel which operates in the Ultra High Frequency band (UHF).
A re-transmission of said command signal may be performed in a said channel which operates in the Low Frequency band (LF).
Said remote transponder may further comprise a remote locking and/or unlocking means having a transmitter stage, wherein said re-transmission of said response signal is performed using said transmitter stage and in at least one said channel used for at least one of said remote locking or unlocking.
Said security control means may be arranged to receive said response signals in any of said range of channels and may further comprise a scanning means arranged in use to scan said range of channels in order to determine in which said channel or channels said response signal or said re-transmitted response signal can be received.
Said remote transponder may be arranged, in response to a detection of a re-transmitted said command signal, to re-transmit said response signal in a first different one of said channels and then in at least a second different one of said channels.
Said different channel or channels in which said response signal is re-transmitted may be substantially preset in said remote transponder.
Said security control means may be arranged to scan said range of channels for a said response signal or a said re-transmitted response signal by tuning itself to each of said channels and remaining tuned to that said channel for long enough to detect the presence of data being transmitted in that said channel.
Said security control means may be arranged to re-transmit said command signal if it does not successfully receive a said response signal within a time slot of predetermined duration which commences on or after an initial transmission of said command signal.
Said response signal may be produced by modulating said command signal and said modulation may be in the order of 2.5% of said command signal.
The security system may comprise the security system of a vehicle.
The invention also provides a method of controlling a security system, the security system comprising a security control means and a remote transponder which are arranged in use to communicate in a range of channels using command and response signals transmitted in one or more of said channels, said security control means being arranged in use to transmit a said command signal in a said channel, said remote transponder being arranged in use, in response to a receipt of said command signal, to transmit a said response signal initially in an initial said channel, said security control means being arranged, if it does not successfully receive a said response signal, to re-transmit said command signal, the method including the steps of:
a) determining if a command signal is a re-transmitted command signal; and
b) re-transmitting, if a said command signal is determined to be a re-transmitted said command signal, said response signal in one or more of said channels which is/are different to said initial channel.
The method may include determining that a particular said command signal is a re-transmitted said command signal if said remote transponder receives that particular said command signal within a time slot of a predetermined duration which commences on or after it has received an earlier said command signal.
The method may include modifying said command signal before re-transmitting it, such that said modified re-transmitted command signal includes information identifying it to said remote transponder as a re-transmitted said command signal.
The method may include indicating in said information which different said channel or channels is or are to be used to re-transmit said response signal in reply to said re-transmitted command signal.
The method may include transmitting said command signal in a said channel which operates in the Low Frequency band (LF).
The method may include transmitting said initial response signal in a said channel which operates in the Low Frequency band (LF).
The method may include, in response to the detection of a re-transmitted said command signal, re-transmitting said response signal in a said channel which operates in the Ultra High Frequency band (UHF).
The method may include re-transmitting said command signal in a said channel which operates in the Low Frequency band (LF).
Said remote transponder may further comprise a remote locking and/or, unlocking means having a transmitter stage and the method may include re-transmitting said response signal using said transmitter stage and in at least one said channel used for at least one of said remote locking or unlocking.
Said security control means may be capable of receiving a said response signal in any of said channels and may further comprise a scanning means capable of scanning said channels, and the method may include scanning said range of channels in order to determine in which said channel or channels said response signal or said re-transmitted response signal can be received.
The method may include, in response to a detection of a re-transmitted said command signal, re-transmitting said response signal in a first different one of said channels and then in at least a second different one of said channels.
The method may include substantially presetting in said remote transponder said channel or channels in which said response signal is re-transmitted.
The method may include scanning said range of channels for a said response signal or a said re-transmitted response signal by tuning said security control means to each of said channels and remaining tuned to that said channel for long enough to detect the presence of data being transmitted in that said channel.
The method may include re-transmitting said command signal if said security control means does not successfully receive a said response signal within a time slot of predetermined duration which commences on or after an initial transmission of said command signal.
The method may include producing said response signal by modulating said command signal and may include modulating said command signal by about 2.5%.